It is often necessary to secure a workpiece or other object to the surface of structures built with concrete and similar materials. This has typically been accomplished by drilling a hole in the concrete, inserting some type of anchoring assembly, setting the anchoring mechanism within the hole (i.e., securing the assembly to the concrete) and securing the workpiece to the anchor assembly. The anchoring mechanism of many such anchor assemblies generally comprises a spreading and anchoring component. In setting, a tapered portion of the spreading component is forced against a portion of the anchoring component, typically having a finger-like structure. The taper causes the fingers to expand or spread either into an undercut cavity formed in the hole below the surface of the receiving material or against the inner walls of the hole itself. A setting sleeve is also typically used to facilitate engagement of these two components by transmitting at least part of the setting force.
A variety of anchoring assemblies of this general type have been devised, but two basic designs seem to be prevalent. With one type of anchoring assembly, the spreading component is secured to the leading end of a bolt or threaded shaft, with the tapered portion pointing back toward the trailing end of the shaft. The anchoring component is located next in line on the shaft, with expandable fingers pointing toward the leading end of the shaft and the tapered portion of the spreading component. During setting, the anchoring fingers expand or spread open toward the bottom of the hole. With the other type of anchoring assembly, the location on the shaft of the spreading and anchoring components are reversed, with the anchoring fingers pointing back toward the trailing end of the shaft. With either design, the setting sleeve is located on the shaft after the spreading and anchoring components. With some of the anchoring assemblies which use the former configuration, this sleeve is an integral part of the anchoring component. With some of the assemblies which use the latter configuration, it is an integral part of the spreading component.
When an anchoring mechanism of either configuration is initially set in a bare pilot hole (i.e., no undercut), spreading of the fingers generates lateral or transverse forces against the walls of the hole. Undercuts in the bored holes have been used, with both configurations, in an effort to reduce these initial lateral forces. The undercuts are typically conical shaped and provide an open area for the anchoring fingers to expand into. However, anchoring assemblies with the former configuration (i.e., fingers pointing toward the leading end of the shaft) still produce very high lateral forces when a subsequent axial load is applied to the assembly (i.e., tensile force applied to the threaded shaft). Depending on the angle of the conical undercut and the tapered portion of the spreading component, these forces can be as high as five times the axial load. These lateral forces can cause lateral bursting of the concrete if the assembly is anchored (i.e., the hole drilled) to close to a side surface of the concrete. In contrast, when a load is applied to an assembly which has the latter configuration (i.e., fingers pointing toward the trailing end of the shaft), most of the forces generated are exerted lengthwise or longitudinally between the ends of the anchoring fingers and the entrance to the hole. Therefore, the likelihood of lateral bursting is much lower (i.e., the hole can be bored closer to a side surface) with the latter configuration.
Despite the lower lateral forces exerted by the latter configuration, there appears to be more widespread acceptance of the former configuration in the construction industry in general, and the nuclear construction industry in particular. Even though the former configuration generates much greater lateral forces, when remotely located from side surfaces, such assemblies have been found to resist axial loading better than previously available assemblies having the latter configuration. In general, a high resistance to axial loading is important in the construction industry. However, it is also desirable, and mandatory in the nuclear construction industry, for the assembly to deform (i.e., absorb energy) under overload conditions rather than fail catastrophically (i.e., brittlely) and to fail before the concrete receiving material fails. In fact, the nuclear construction industry requires the threaded shaft to fail before the anchoring mechanism or the concrete.
There are times when it would be desirable to remove and replace the threaded shafts and leave the anchoring mechanism in place (e.g., when threads are damaged, the bolt is broken, etc.). This versatility would also permit the anchoring mechanism to be set to 100% of the ultimate strength of the threaded shaft by first tensioning the anchoring mechanism with a higher rated (i.e., stronger) shaft and then replacing that shaft with one having the appropriate characteristics (i.e., strength, corrosion resistance, etc.) for the particular application. With previously available anchoring assemblies, there is a significant risk of disengaging the anchoring and spreading components during removal of the shaft. This could require the assembly to be reset or even prevent resetting (e.g., if one of the components fell to the bottom of the hole). In addition, in some anchoring assemblies, the shafts cannot be separately removed at all. If the same threaded shaft is used for setting the anchoring mechanism and securing the workpiece, then the applied force used for setting must be below the threaded shaft's yield strength or risk damaging the shaft itself. Thus, it could not be guaranteed that the shaft would fail before the anchoring mechanism or the receiving material.
When a hole is drilled or bored, it is not always possible to guarantee the hole's depth. If the hole is drilled deeper than the depth at which the anchoring mechanism is to be located, a number of anchoring assemblies could not be used, notably the assembly disclosed in Liebig, U.S. Pat. No. 4,690,597. In Liebig, the anchoring mechanism is set by seating the anchoring component at the bottom of the hole, with its fingers pointing toward the entrance of the hole, and driving the tapered portion of the spreading component down toward the anchoring component, spreading the fingers. Thus, with such anchoring assemblies the depth of the hole must correspond with the proposed location of the anchoring mechanism.